The present invention relates to a fabric of low permeability suitable for use in an air bag and, more particularly, to such a fabric formed of an uncoated woven cloth.
Low permeability fabrics have a multitude of potential uses, e.g., air-filled or water-filled mattresses, sleeping bags, pillows, life belts, life boats, chairs, ottomans, etc. Each of these alternative uses may have a different maximum permeability requirement.
Among the most demanding and critical of this multitude of potential uses is their employment in expansible passenger restraint bags for automobiles ("air bags"). The air bags are adapted to be inflated instantaneously with high pressure gas in the event of a collision to prevent the movement of the occupant for safety reasons. For this purpose, the air bag is generally made of an air-impermeable material and formed with an opening from which the high pressure gas introduced into the bag is exhausted, whereby the energy resulting from the occupant's striking against the bag upon a collision and the energy of the subsequent pressing movement are absorbed to reduce the impact. This potentially life-saving use requires that the fabric exhibit not only low permeability, but also be strong, lightweight, thin and flexible so that it can be easily and compactly folded into its appropriate storage container ready for use, withstand the strong shock occurring when it is initially deployed, and add as little weight as possible to the person or vehicle transporting it.
In the past, fabrics intended for use in air bags have been made from a wide variety of materials including macromolecular film (such as polyethylene) and inorganic fibers (such as glass fiber).
Such patents as U.S. Pat. No. 3,705,645 and U.S. Pat. No. 3,892,425 illustrate the conventional manner of forming such fabrics from a woven cloth by coating the cloth with a resin binder to reduce its permeability. Low permeability fabrics for use in air bags are conventionally made of nylon or polyester which has been coated with neoprene, urethane, or silicon resin, even though the coating necessarily increases the weight, thickness (bulk), stiffness, and cost of the cloth while reducing its flexibility, tear strength, over-all strength (due to prolonged exposure to heat during the coating operation) and shelf-life (as the coatings tend to degrade over time). Variations in the coating thickness from lot to lot or within a single lot can also introduce undesirable permeability fluctuations for a given piece of fabric.
U.S. Pat. No. 3,730,551 discloses a knit or woven nylon or polyester material suitable for use in an air bag, but there is no specific disclosure of the permeability of the fabric or precisely how the fabric is woven (or coated) to provide a suitable permeability according to 1971 standards.
In any case, air bag manufacturers have recently proposed a new and extremely rigorous requirement of low permeability for an uncoated fabric not exceeding one cubic foot of air per minute per square foot of cloth (0.5 cm.sup.2 /sec/cm.sup.2) at a pressure drop of 0.5 inch of water (1.27 cm) across the cloth (i.e., 1 CFM). In addition, an air bag fabric should be flexible, this, preferably with a thickness of less than 0.016 inch (0.041 cm), lightweight, preferably with a weight of not more than 8.25 oz/square yard (280 gms/m.sup.2), and strong, preferably with a strength characterized by a tensile strength of at least 300 lbs (1334 newtons), a Mullen burst strength of at least 650 psi (4482 kilopascals), and a trapezoid tear of at least 40 lbs. (178 newtons). Preferably the fabric should also be flexible, compactable, have an elongation at break of at least 25%, and exhibit a storage or shelf life of 5 to 10 years without substantial deterioration.
A strong, lightweight thin, flexible fabric has been sold for use in bomb parachutes as a bomb parachute (for the controlled descent of a bomb, rather than a person), but this fabric was characterized by a permeability of about 1.5-2.0 CFM (specification maximum 3.0 CFM) and thus would not meet the aforenoted extremely low permeability requirement of the proposed new standard for an air bag fabric. Further, the fabric, made of an uncoated, Woven nylon calendered on one side to reduce permeability (210/34/0 multifilament yarn woven in a 1.times.2 modified Oxford weave, 80 ends/inch.times.80 picks/inch), exhibited low flexibility and hence poor compactability.
Thus, the need remains for a fabric which is strong, lightweight, thin and flexibile, yet of extremely low permeability.
Accordingly, it is an object of the present invention to provide a strong, lightweight, thin, flexible fabric having a permeability of not more than 1 CFM.
Another object is to provide such a fabric which is suitable for use in air bags.
A further object is to provide a method of making such a fabric or a similar one having a permeability of not more than 3 CFM.